Process for preparing a dry mixture of ammonium fluosilicate and silica



PROCESS FOR PREPARING A DRY MIXTURE OF AMMONIUM FLUOSILICATE AND SILICA No Drawing. Application August 4, 1954, Serial No. 447,905

8 Claims. (Cl. 23-88) This invention relates to a proceess for the preparation of ammonium fluosilicate and silica. In one aspect, it relates to the preparation of dry ammonium fluosilicate and silica from silicon tetrafluoride. In another aspect, it relates to a process for the vapor phase elimination of toxic fluorine-containing constituents of the off-gases of commercial mineral acidulation processes, e. g., the phosphate rock acidulation process, and recovery of the gaseous fluorine values as dry, finely divided ammonium fiuosilicate.

Many naturally occurring materials contain small quantities of silica and fluorine chemically associated with other elements or with themselves. On acid treatment of these materials, much of the fluorine is liberated as silicon tetrafluoride. An example of this is the acidulation of phosphate rock to form superphosphate or phosphoric acid. This mineral contains about 3-4 percent of chemically combined fluorine, at least -40 percent of which is evolved in the sulfuric acid acidulation of the rock. The off-gas stream contains air, water vapor, carbon dioxide, small quantities of other volatile constituents of the rock and about /2-2 percent by volume of silicon tetrafluoride. These off-gases from the superphosphate and phosphoric acid processes represent the cheapest known source of silicon tetrafluoride. However, separation of silicon tetrafluoride in a pure form from the gas stream is technically and economically impractical.

The fluorine values are presently recovered by scrubbing the silicon tetrafluoride-containing off-gas stream with water to form silica and fluosilicic acid of concentrations betwen about 20 percent and 28 percent by weight H2SiFa. Approximately half of the silica formed is easily filtered from the crude fluosilicic acid, and the balance of the silica in very finely divided or colloidal form remains suspended in the filtered acid. Water soluble impurities contained in the gas stream are, to a large extent, adsorbed in the scrubbing towers and contaminate the fluosilicic acid product. The fluosilicic acid solution may be treated with metal salts to form corresponding metal fluosilicates.

It has also been proposed to pass the fluorine-containing off-gases evolved during evaporation of phosphoric acid into aqueous solutions of ammonia or ammonium salts, whereby a solution of ammonium fluosilicate is formed and silica settles to the bottom of the adsorption apparatus. This method, as well as the fluosilicic acid method, requires a series of scrubbing towers or equiva' lent adsorptive apparatus, with the attendant solution handling problems. Both methods recover the fluorine in aqueous solution. The ammonium or metal fluosilicate solutions may be crystallized and the fluorine values ultimately recovered in a dry state. The necessary evaporation of course entails additional apparatus and expense. Furthermore, the undesirable impurities in the off-gas stream are retained in the ultimate aqueous fluorine-containing solution.

2,785,953 Patented Mar. 19, 1957 ice In accordance with the present invention, it is now possible to effect a dry recovery of the fluorine values in a silicon tetrafluoride-containing gas mixture.

It is, therefore, an object of this invention to provide an improved method for recoveri g fluorine values in the oil-gases of the phosphate rock acidulation process. It is another object to provide a method of preparing ammonium fluosilicate and silica from silicon tetrafluo ride-containing gases. Other objects and advantages of the present invention will be apparent to those skilled in the art from the following detailed description and the appended claims.

The process of the present invention comprises contacting silicon tetrafluoride in the presence of water vapor with gaseous ammonia at elevated temperatures to form a finely divided white powder comprising ammonium fluosilicate and silica. The reaction involved in the present invention proceeds according to the following equation:

This reaction is carried out at temperatures above the condensation point of water in the reaction mixture and below about 300 C.

The present invention is particularly well suited to the recovery of fluorine values from the silicon tetrafluoridecontaining oti-gases of the phosphate rock acidulation process. The invention, however, is not limited to these off-gases, but is equally well adapted to the conversion of substantially pure silicon tetrafluoride to dry, solid ammonium fluosilicate and silica. The oil-gases of many commercial phosphate rock acidulation processes are very dilute with respect to silicon tetrafluoride, being within the range of only about /2 to 2 percent by volume. These oil-gases, which leave the acidulation apparatus at about 71 (3., contain more than suflicient water vapor to satisfy the above equation. In the case of a dry silicon tetrafluoride starting material, suflicient water vapor must be added to the system to at least satisfy the equation. The present invention is particularly well adapted to the treatment of gases very dilute with respect to silicon tetrafluoride because its reaction with water vapor and ammonia is almost instantaneous.

The present process is carried out at temperatures above the condensation point of water in the gaseous reaction mixture and preferably below about 250 C. The lower practical temperature limit is governed by the fact that silicon tetrafluoride upon contact with liquid water in the presence of ammonia reacts to form siliceous deposits and soluble ammonium fluoride salts. Thus, the reaction involved in the present invention may be conducted at temperatures below the boiling point of water so long as condensation of water is avoided; The reaction illustrated by the above equation proceeds very nearly quantitatively up to about 250 C. with stoichiometric quantities of ammonia. At temperature above about 250 C. the reaction is considerably retarded. However, in the presence of excess ammonia the upper temperature limit is raised 25 to 50 C. The reaction is exothermic and this fact must be taken into consideration in choosing the temperature at which the reaction is to be conducted.

The product of the present invention is a dry, white powder analyzing approximately 86% (NH-t)zSiFs and 14% SiOz. The agglomerates generally are in the range of 1 to 5 microns. As observed by the electron microscope, the ultimate particles range from 50 to 500 millimicrons in diameter. The solid product is easily separated from the gaseous reactants by means of a bag or other equivalent filtering means.

The present invention is further illustrated in the following non-limiting examples.

Example I from runs 1 and 2, conducted with stoichiometric quan- The reaction between silicon ten-fluoride water vapor t1t1es of ammonia and silicon tetrafluoride, failed to reand ammonia was examined at Various temperatures veal the presence of ammonia, which indicates quantitatween 100 c. and 250 c. A 30-inch glass tube 1-inch five recovery of ammonia in the Product The q y i di was id d i h i bl Stoppers at h 5 of the product recovered in runs 6 and 8 was not sufliclent intake and exit ends. The tube was fitted with two glass in Permit analysis thereof for m n hi accounts plugs so as to provide two dj Small comharttor the absence of these figures in the above table Withments of approximately equal length near the intake end. experimental limits, the product Of ailh run had a The remaining length of tube constituted the reaction m ratio of NHfiZSlOZiF of 12, the equation 1'11- zone, A i l glass wool plug was placed at the i 10 dtcates. X-ray defraction analysis of the powder prodd of th b dj h stopper [0 prawn; 1 f uct showed the same pattern of lines as did commercial th d t m th i gas Stream hi h was d d ammonium fluosilicate, providing positive identification through an exhaust i e In the stopper The intake stop- In Order to test reaction efficlency and Product p y terminated in jets for the introduction of moist air, siliapparatus was Set P and the Off-gases Of a C mmercial con tetrafiuoride, and an ammonia or an ammonia-air Phofiplllilc F flcidulflllonpl'ocess were p y as the mixture into the reaction zone. Moist air saturated with 5mm? of 511190" ietrafluondewater vapor at 75 C. was introduced into the first com- Example [I i e r i t ti riii t litj m ri'it vii zi i' irlifri m aii igii ui i g A gallon drum was lagged with asbestos, partially filled with water, placed on a hot plate and used as a steam bath. A stoppered three quart bottle was suspended in the drum and served as the reaction vessel. Its stopper was adapted to pass two /8 in. pipes, the off-gas and ammonia feeds, as well as a A in. exhaust pipe ranged adjacent the length of the reaction tube The off-gases, leaving the ac1dulat1on apparatus at a temperature of about 160 F. contain on the average A series of e1ght 90-minute runs was conducted at about 1/2 1 percent Sim 3 percent CO2 and 13 percent various reaction temperatures w1th an excess of water H2O by volume, as We as air and small quantities of vapor. Two runs were carried out with stoichiometric quantities of silicon tetrafluoride and ammonia, and the $2 g i g dust The dew p 01m of.these gases remaining runs conducted with varying degrees of excess ammonia. A visible reaction occurred readily, and with stoichiometric quantities of ammonia no excess appeared in the exit gases. At the ammonia jet a dense white cloud formed immediately which settled slowly throughout the length of the reaction zone. Upon continued operation with undiluted ammonia, a plug of solid product formed at the ammonia jet. Plugging was prevented s by ammonia e metast ses? assets: with L3 parts per part of ammonia The reaction 4 vessel was maintained at a temperature of at least 100 appeared to be instantaneous at temperatures of from C by steam generated in the drum 532 g ff gg fii g .h fgg 'jg z ggi The white finely divlded product (SiO2+2(NHt)zSiFe-) was collected at the exhaust pipe filter and the bottom of the reaction vessel. Five hours of continuous operation 40 the second feed pipe at a rate sufi'icient to maintain an gas collection system. SO Paesrcgeit h r l wing table sets out reaction conditions, weight 2 49.75 and n y is of the product, as well as the me] ratio of 5; 15.60

ammonia to silica to fluorine in the solid product for this series of runs.

Product Per- Per- Per- Molc Ratio,

Percent Av. Mole Ratio, Tgrnp, '15., cent cent cent V01. 31F NHatSiF tHgO O SiOz NIT i F NHUSIOMF M n i 2?. 5+0 2. 0 1.38:1:20. 2 11. 4 s9. s1; 2s.a+e5 2.2 1.17:1:1as 100 10.6 37.78 37+0 1. 4 a. 05:1:25 100 7. 5 a9. 49 1111+1311 2v 5 4. 53:1:9. 7 14. 4 as. 26 109+12a 1.8 5.53:1:117 16.8 38.95 1a1+120 0.61 l9.9:1:50.8 200 4 41.33 107+114 (1.76 13.2:1.-41 225 6.6 42.00 1

0. as 16. 6:1:48 250 5. a 40. 32 j g A sample of the dry finely divided white powder pre- F6203 pared at 100 C. was leached with water and found to C03 00 contain 14.2% SiO2 by weight which agrees favorably P205 with a silica content of 14.4% calculated for the mixture The mole ratio of NHmSiOzrF in the prod Sl03+2(NH4)2SlF6. An analysis of the exhaust gases 75 4.31.-3=12.41.

The present invention thus provides a particularly eflicient method for recovering fluorine values from the oilgases of the phosphate rock acidulation process. The invention also provides an equally eificient method for the conversion of silicon tetrafluoride to a dry very finely divided mixture of ammonium fiuosilicate and silica. The advantages of the present dry process over the methods of the prior art are manifest from the nature of gas phase reactions generally and the obvious apparatus and circulation limitations of any liquid-gas adsorption proc ess.

The silica impurity, equal to approximately 14% by weight, will not in most instances be a deterrent to use of the product in processes heretofore employing pure ammonium fluosilicate. For example, because of the very dilute concentrations employed, silica will not cause turbidity or adversely affect the clothes being treated when the material is used as a laundry sour. The product may also be used in the magnesium casting art, where ammonium fluosilicate is added to the sand molds to prevent oxidation and scaling of the casting.

On the other hand, the product may be leached with water to remove silica and the resulting aqueous ammonium fiuosilicate solution concentrated for use or pure ammonium fluosilicate crystallized therefrom. Alternatively, ammonia may be added to an aqueous solution of ammonium fluosilicate to a pH of about 89, silica filtered from the solution, and substantially pure ammonium fluoride recovered. The thus formed silica is exceedingly finely divided and has a number of obvious uses.

The product is also an excellent source of cheap substantially pure silicon tetrafluoride. This compound may be recovered by treating the product with concentrated sulfuric acid. Silicon tetrafluoride is evolved while ammonium sulfate and ammonium acid sulphate remains dissolved in the sulfuric acid solution. The resulting sulfuric acid may be employed in the acidulation of phos phate rock, thus utilizing the dissolved ammonium salts as a source of nitrogen in the superphosphate formed.

The present invention also provides a vapor phase process for the preparation of primary amine fluosilicates. The amine is substituted for ammonia in the reaction and the solid product contains one mole of silica and two moles of amine fiuosilicate. Separation may be effected by water leaching as in the case of the silicaammonium fiuosilicate product. Isopropylamine and butylamine are particularly reactive.

I claim:

1. A process for preparing a dry mixture of ammonium iluosilicate and silica which comprises admixing silicon tetrafluoride with at least two-thirds mole of water vapor and at least four-thirds moles of gaseous ammonia per mole of silicon tetrafluoride at temperatures above the condensation point of water in the gaseous mixture and below about 300 C., thereby forming a dry mixture of finely divided ammonium fiuosilicate and silica in an atmosphere free of liquid water and separating said dry mixture from said atmosphere above the condensation point of water.

2. A process for preparing a dry, finely divided mixture of ammonium fluosilicate and silica which comprises admixing silicon tetrafluoride with at least two-thirds mole of water vapor per mole and at least 1.33 moles of gase ous ammonia per mole of silicon tetrafiuoride reacting at temperatures between about 80 C. and 250 C., and separating the finely divided mixed product from the gaseous reactants above the condensation point of water.

3. An improved process for recovering fluorine values in substantially pure form from a gas mixture containing impure silicon tetrafluoride and water vapor in a ratio of at least two-thirds mole of water vapor per mole of silicon tetrafluoride, which comprises adding to said gas mixture at least 1.33 moles of gaseous ammonia per mole of silicon tetrafluoride at temperatures above the conden sation point of water in the gaseous reaction mixture and Cir till

6 below about 250 0., thereby forming a dry, finely divided mixture of ammonium fluosilicate and silica, filtering said finely divided mixture from the gaseous at a temperature above the condensation point of water, and water leaching said mixture to form a solution of ammonium tluosilicate substantially free of silica and the contaminants of the silicon tetrafluoride-containing gas mixture.

4. A process for preparing a dry, finely divided mixture of ammonium fiuosilicate and silica which comprises admixing silicon tetrafluoride-containing gas containing at least two-thirds mole of water vapor per mole of silicon tetrafiuoride with at least four-thirds moles of gaseous ammonia per mole of silicon tetrafiuoride at a temperature above the condensation point of water vapor and below about 250 C., and filtering the resulting finely divided mixture of ammonium fiuosilicate and silica from the gaseous reactants above the condensation point of water.

5. A process for preparing a dry, finely divided mixture of ammonium fiuosilicate and silica which comprises admixing oil-gases of the phosphate rock acidulation process containing silicon tetrafluoride and at least twothirds mole of water vapor per mole of silicon tetrafiuoride with at least four-thirds moles of gaseous ammonia per mole of silicon tetrafluoride at temperatures above the condensation point of water in the gaseous reaction mixture and below about 250 C., and filtering the finely divided ammonium fiuosilicate and silica product from the gaseous reactants above the condensation point of water.

6. A process for obtaining substantially pure silicon tetrafiuoride from oil-gases of the phosphate rock acidulation process containing silicon tetrafiuoride and at least two-thirds mole of water vapor per mole of silicon tetrafluoride, which comprises filtering said oil-gases to remove entrained solids, admixing said filtered off-gases with at least 1.33 moles of gaseous ammonia per mole of silicon tetrafiuoride at a temperature above the condensation point of water in the gaseous reaction mixture and below about 300 0., thereby forming ammonium fluosilicate and silica in admixture as dry solids, filtering said finely divided mixture of ammonium fiuosilicate and silica, substantially free of the impurities of said offgases, from the gaseous reactants, introducing said solid mixture into concentrated sulfuric acid and collecting the substantially pure silicon tetrafiuoride evolved.

7. A process for recovering fluorine values from a. gaseous mixture containing silicon tetrafiuoride comprising adding to said mixture at least two-thirds mole of water vapor and four-thirds moles of ammonia per mole of silicon tetrafluoride, thereby inducing the formation of ammonium fiuosilicate and silica in admixture as a finely divided solid and filtering said admixture of ammonium fluosilicate and silica from said gaseous mixture, said formation reaction and recovery being carried out at a temperature above the condensation point of water in the system and below about 300 C.

8. A process for forming ammonium fiuosilicate comprising adding to a silicon tetrafiuoride-containing gas at least two-thirds mole of water vapor and four-thirds moles of gaseous ammonia per mole of silicon tetrafluoride, maintaining the temperature of the resulting re action mixture above the condensation point thereof but below about 300 0., thereby forming silica and ammonium fiuosilicate as a mixture of dry, finely divided solids, and filtering said finely divided solids from said reaction mixture at a temperature above the condensation point of the system.

References Cited in the file of this patent UNITED STATES PATENTS 1,235,552 Chappell Aug. 7, 1917 1,859,998 Svendsen May 24, 1932 1,911,004 Svendsen May 23, 1933 2,447,359 Oakley Aug. 17, 1948 

1. A PROCESS FOR PREPARING A DRY MIXTURE OF AMMONIUM FLUOSILICATE AND SILICA WHICH COMPRISES ADMIXING SILICON TETRAFLUORIDE WITH AT LEAST TWO-THIRDS MOLE OF WATER VAPOR AND AT LEAST FOUR-THIRDS MOLES OF GASEOUS AMMONIA PER MOLE OF SILICON TETRAFLUORIDE AT TEMPERATURE ABOVE THE CONDENSATION POINT OF WATER IN THE GASEOUS MIXTURE AND BELOW ABOUT 300*C., THEREBY FORMING A DRY MIXTURE AND FINELY DIVIDED AMMONIUM FLUOSILICATE AND SILICA IN AN ATMOSPHERE FREE OF LIQUID WATER AND SEPARATING SAID DRY MIXTURE FROM SAID ATMOSPHERE ABOVE THE CONDENSATION POINT OF WATER. 